The invention relates to a process for the low-temperature fractionation of air in which compressed and prepurified feed air is introduced into: a rectification system for nitrogen-oxygen separation, which rectification system has a pressure column, where at least a part of the compressed and prepurified feed air is fed to the pressure column and where an oxygen-enriched fraction is taken off from the pressure column and passed to a further working step within the rectification system.
Processes of this type are disclosed, for example, by Hausen/Linde, Tieftemperaturtechnik [low-temperature engineering], 2nd edition 1985, Chapter 4 (pages 281 to 337). The rectification system for nitrogen-oxygen separation can be a one-column system having a single column, the pressure column in the context of the invention, a two-column system having a pressure column and a low-pressure column or a multi-column system. having further separation columns for nitrogen-oxygen separation. Hausen/Linde shows a plurality of examples of one-column systems (page 282, FIGS. 4.1. and 4.2.; page 287, FIG. 4.4; pages 329/330, FIGS. 4.30., 4.31. and 4.32.); the invention can be applied especially to a single column with top cooling via an oxygen-enriched liquid from the pressure column (Hausen/Linde, page 330, FIG. 4.31). Examples of two-column systems may also be found in Hausen/Linde (page 284, FIG. 4.3. and various examples in Sections 4.5.1 and 4.5.2). To generate ascending vapour for the low-pressure column, a part of the bottom liquid is evaporated in a condenser-evaporator (usually termed main condenser) which is operated, for example, with a gas fraction from the pressure column or with air as heating medium. The condenser-evaporator can be implemented by one or more heat-exchange blocks which can be operated, for example, as circulation evaporator and/or falling-film evaporator.
The rectification system for nitrogen-oxygen separation in the context of the invention in addition comprises heat exchangers such as, for instance, condenser-evaporators which are required to operate the separation column(s) for the nitrogen-oxygen separation (especially the main condenser of a double column or the top condenser of a single column). The process of the invention and the corresponding apparatus can, if required, have, outside the rectification system for nitrogen-oxygen separation, additional separation columns for producing further air constituents, for example noble gases such as argon, helium, neon, krypton or xenon (see Hausen/Linde, Chapter 4.5.4).
Customarily, the oxygen-enriched fraction is taken off from the bottom region of the pressure column before it is passed to a further working step within the rectification system. This further working step can be formed, for example, by the further separation in the low-pressure column of a double-column system or by an evaporation, for example in the top condenser of a one-column system. By this means, all of the low-volatility contaminants of the feed air which were not removed in the prepurification upstream of the introduction into the rectification system are transported on into the subsequent working step together with the oxygen-enriched fraction. (xe2x80x9cLow-volatility contaminantsxe2x80x9d is here taken to mean feed-air components whose boiling point is higher than that of oxygen.)
Such low-volatility contaminants can accumulate further, especially in subsequent evaporation processes. Some of these low-volatility substances, in particular N2 O, can precipitate as solids and must be removed from time to time to avoid a blockage of heat-exchange passages in the corresponding evaporators (for example in the main condenser of a double-column system). To eliminate the solids which have settled out, the entire plant must be shut down. In the case of a large air fractionation plant, this can mean an operational shutdown of, for example, two to five days. This problem is discussed in Wenning, Lachgas in Luftzerlegungsanlagen [laughing gas in air fractionation plants], Linde-Berichte aus Technik und Wissenschaft, 77/1998, 32-36. Here, and in U.S. Pat. No. 5,629,208 the solution proposed is to eject N2O using a relatively intense flushing of the liquid arising in the main condenser. However, it has proved to be the case that this measure is not sufficient in all cases to avoid the operationally highly undesirable shutdown of the air fractionation plant.
As a solution to this problem, various methods available to those skilled in the art seem suitable.
Firstly, a purification device could be used which removes the unwanted substances from the oxygen-enriched fraction. In the course of this, for example, all of the oxygen-enriched fraction (the bottom-phase liquid of the pressure column in the case of a double column) is conducted in liquid form via an adsorber to remove N2O. (Liquid adsorbers were previously used at the same point to remove acetylene.) This procedure solves the operational problems in the evaporator, but represents relatively high capital expenditure. In addition, the adsorber must be regenerated from time to time, which, even in the case of a switchable device, leads to further operational expenditure.
Secondly, U.S. Pat. No. 5,471,842 discloses ejecting low-volatility components as early as in the pressure column by withdrawing, at its bottom, a purge fraction in the liquid state and by taking off, above the air feed, the oxygen-enriched fraction to be processed further in the low-pressure column. The purge fraction here is brought in the liquid state to a very high pressure, vaporized in the main heat exchanger against highly compressed feed air, admixed to the feed air upstream of the air prepurification and passed with this feed air back into the pressure column. Although this method works, as specified in U.S. Pat. No. 5,471,842, for the ejection of CO2 which is effectively retained in the prepurification molecular sieve, the problem of N2O is not mentioned in U.S. Pat. No. 5,471,842. The process there is unsuitable for reliable ejection of N2O (see article by Wenning, paragraph 6); under some circumstances blockage of the passages of the main heat exchanger can occur due to precipitated N2O, which necessitated heating of this apparatus.
In a modification of the process proposed in U.S. Pat. No. 5,471,842 for CO2 ejection, it would be possible to remove from the process completely the purge fraction withdrawn in the bottom of the pressure column by discarding it, if appropriate after recovery of some of its cold. The purge fraction can, for example, be directly discarded in the liquid state by discharging it, after removing it from the pressure column, into the atmosphere, for example via an ejector. Alternatively, it can be vaporized and/or heated by indirect heat exchange with a heating medium and then discarded in the gaseous state. By this means, some of the energy which is present in the purge fraction in the form of cold is recovered. The vaporization should take place at a temperature high enough that precipitation of low-volatility contaminants is avoided, for example by introducing the liquid purge fraction into a residual gas fraction at medium temperature. Another possibility is the recovery of the cold in a heat exchanger having switchable passages (Revex). All of these methods can be expedient in certain plants, but have the disadvantage that the separation work performed on the purge fraction is lost and thus there is a high operating expenditure in the form of additional energy consumption.
The object therefore underlying the invention is to design a process of the type mentioned at the outset and a corresponding apparatus in such a manner that the operating expenditure in the overall process can be kept particularly low.
This object is achieved by the features of Patent claim 1. In the process of the invention the purge fraction, which is formed by at least a part, preferably the whole, of the bottom liquid of the pressure column, is fed without prior vaporization to a device for removing N2O.
As a result, the purified purge fraction can be fed downstream of this device to further working steps within or outside the rectification system for nitrogen-oxygen separation, without the accumulation of N2O threatening within the context of these working steps. The further working step can have, for example, a column for nitrogen-oxygen separation or a condenser-evaporator for generating reflux for such a column, for instance the low-pressure column of a two-column system for nitrogen-oxygen separation or the top condenser of the pressure column.
The mass transfer section between the point of the feed air supply (generally at the bottom of the pressure column) and the takeoff of the oxygen-enriched fraction permits a largely complete scrubbing of the low-volatility contaminants, in particular N2O, from the feed air into the bottom phase of the pressure column. The mass transfer section is formed either by at least one actual plate or by a packing section having a separation action of at least one theoretical plate. Preferably, there are from 1 to 10, highly preferably from 3 to 5, theoretical or actual plates between air feed or pressure column bottom on the one hand and takeoff point of the oxygen-enriched liquid on the other hand. (In the event that solely actual plates are used in this section as mass-transfer elements, the figures apply to numbers of actual plates; if an arranged packing, dumped packings or combinations of different types of mass-transfer elements are-used, the figures apply to numbers of theoretical plates.)
In the context of the process of the invention, the pressure column can be implemented as a single vessel. As an alternative, different sections can be enclosed by separate vessels. For example, the mass-transfer section which serves for scrubbing out N2O can be constructed separately from the rest of the pressure column (see apparatus according to Patent Claim 11).
With a mass-transfer section of this type between air feed and takeoff of the oxygen-enriched fraction, the most important low-volatility contaminants can be held back virtually completely from subsequent working steps. The oxygen-enriched fraction, for example, comprises less than 1 ppb of N2O (molar concentration less than 1031 9), preferably the molar N2O concentration is 1031 12 or below.
The low-volatility contaminants such as N2O are removed with the liquid purge fraction from the bottom of the pressure column. The purge fraction can be taken off continuously or batchwise. The amount of purge fraction withdrawn is determined by the wanted or permitted concentration of low-volatility components in the purge fraction. Generally, it is set so that no solids precipitation occurs in the bottom of the pressure column; under some circumstances, however, a higher enrichment with solids precipitation is possible. The amount of purge fraction is, for example, at least 0.1 mol % of the amount of feed air fed into the pressure column, preferably 0.15 mol % to 10 mol %, very highly preferably 0.3 mol % to 5 mol %, of the amount of feed air. (The figures on the amount of purge fraction are, especially in the case of batchwise takeoff, to be understood as an average over time of the amount of purge fraction.)
As a side effect of the measures of the invention, there is established an improvement in product quality of the oxygen product which may be generated from the oxygen-enriched fraction.
Preferably, N2O is removed from the liquid purge fraction in the purification stage by physical adsorption. The purification stage is therefore formed by a liquid adsorber. This liquid adsorber can be designed to be substantially more compact than the liquid adsorbers which have been used previously for acetylene removal and via which was passed the entire oxygen-enriched fraction.
Alternatively, the N2O can be precipitated in a heat exchanger provided separately for this, by evaporating the liquid purge fraction in the purification stage by indirect heat exchange, N2O precipitating out as solid and/or liquid during the evaporation. They can be deposited in the heat exchanger in which the evaporation is carried out. The evaporation must be carried out in this case batchwise or in a switchable pair of recuperative or regenerative heat exchangers, so that the deposited solids can be removed at certain time intervals. However, it is also possible to take off liquid or solids arising and the purified purge fraction continuously.
Another possibility is to remove N2O from the purge fraction in the purification stage by counter-current mass transfer. In this case the purge fraction is introduced in the liquid state into an additional separation column, for example at an intermediate point or at the top. The bottom fraction of the separation column is, for example, discarded, while the overhead fraction is further processed, for example in the pressure column. Heat must be supplied to the bottom of the separation column, for example by indirect heat exchange with a warm stream (transfer of sensible heat) or with a condensing gas stream of suitable composition using an electrically operated heater. In the event that the purge fraction is not applied directly at the top, in addition top cooling is necessary, for example by indirect heat exchange with a vaporizing process stream of suitable composition and suitable pressure.
Generally, one of the three methods for N2O removal is employed. In principle, a combination of two or three variants is also possible; in an example case, the purification stage has both at least one adsorption bed and also at least one switchable pair of heat exchangers.
Alternatively or in addition to the abovementioned introduction into the rectification system for nitrogen-oxygen separation, the purified purge fraction can be fed at least partially to a working step outside this rectification system. Preference is given in this case to feeding it into a system for producing a noble gas, for example krypton and/or xenon, by rectification. Examples of systems of this type can be found in the earlier German patent application 19823526.7 and in the applications corresponding thereto of the same applicant, and also in EP 96610 A, EP 222026 A, DE 166763.9 A, DE 1122088 B or in Streich et al., Gewinnung von Edelgasen in Luftund Ammoniakanlagen [production of noble gases in air and ammonia plants], Linde-Berichte aus Technik und Wissenschaft, 37/1975, 10-14. The purified purge fraction is in this case preferably introduced in the liquid state at least partially into an exchange column which serves for incorporating krypton and xenon into an inert gas (nitrogen or argon). This exchange column can in addition receive the customary krypton- and xenon-containing feed, that is to say the liquid bottom fraction from the low-pressure column of a two-column system.
Preferably, in the process of the invention, the entire air, that is to say the entire feed air which is fractionated in the rectification system, is introduced into the pressure column. The entire feed air is preferably fed into the pressure column at least one theoretical or actual plate below the point at which the oxygen-enriched fraction is taken off. This thus avoids, via direct feed of air into further working steps within the rectification system (for example via an air turbine which leads to the low-pressure column of a two-column system), unwanted low-volatility contaminants from passing into a working step downstream of the pressure column.
In the context of the invention it is expedient if process refrigeration energy is produced by work expansion of an intermediate fraction which is taken off from the pressure column above the air feed. The takeoff point can be, for example, at the intermediate point at which the oxygen-enriched fraction is taken off, at the top of the pressure column, or at any point arranged between these two points. The intermediate fraction is virtually N2O-free and can therefore, downstream of the work expansion, be fed to the low-pressure column.
Alternatively, or additionally, a part of the compressed and prepurified air can be branched off upstream of the pressure column and work-expanded; the expanded air, however, must not then be fed to the pressure column above the air feed or to a working step of the rectification system downstream of the pressure column, but is, for example, admixed to a residual stream and removed from the process.
The production of refrigeration energy can be increased by a pressure increase in the intermediate fraction. For this purpose, the intermediate fraction can be taken off from the pressure column, for example in the gaseous state, upstream of the work expansion, warmed and compressed in the gaseous state. It is expedient, for this compression, to use at least some of the mechanical energy which is produced in the work expansion. The pressure downstream of the compression is, for example, from 7 to 15 bar, preferably from 8 to 12 bar. The height of the pressure difference here depends, as in the paragraph below, on the refrigeration requirement of a specific plant.
Alternatively, the intermediate fraction is taken off from the pressure column in the liquid state upstream of the work expansion, subjected in the liquid state to a pressure increase, vaporized and warmed by indirect heat exchange. The liquid pressure increase leads to a pressure of, for example, from 7 to 15 bar, preferably from 8 to 12 bar.
Deviation from the customary procedure is also expedient in the event that the process of the invention is operated in association with an internal compression process in which a product stream is brought in the liquid state to pressure (for example from 7 to 50 bar, preferably from 9 to 30 bar) and is then vaporized against a heating fluid under high pressure (for example from 7 to 50 bar, preferably from 9 to 30 bar). (The pressures depend in the individual case on the product pressure required.) Instead of a part of the compressed and prepurified feed air, according to another variant of the invention, a virtually N2O-free gas fraction from the pressure column is used as heating fluid. This is taken off at least one theoretical or actual plate above the point at which the compressed and prepurified feed air is fed, preferably at the intermediate point at which the oxygen-enriched fraction is taken off, at the top of the pressure column, or at a point disposed between these two points. The heating fluid is warmed, compressed and finally condensed against the product stream which is brought to pressure in the liquid state. The condensate is further processed at a suitable point, for example in the pressure column.
In the process of the invention, a part of the bottom liquid of the pressure column can be vaporized and the resultant gas can be passed back to the pressure column. This optional bottom heating of the pressure column is preferably effected by a condenser-evaporator which receives a flow of suitable process gas as heating medium. In this manner the throughput is increased in the section of the pressure column which is below the takeoff point of the oxygen-enriched fraction. By this means other substances, in particular krypton and/or methane are extracted into the bottom of the pressure column. This effect is further intensified if the pressure column has in this case a further mass-transfer section which is arranged below the point at which the compressed and prepurified feed air is introduced into the pressure column and has the scope of some theoretical plates.
The invention further relates to an apparatus for the low-temperature fractionation of air according to Patent Claim 11 or 12.
In particular in the retrofitting of existing plants with the process of the invention it can be expedient not to rebuild the column(s) on existing rectification system, but to use an additional precolumn which comprises the mass-transfer section between air feed and takeoff of the oxygen-enriched fraction. The pressure column in the context of the invention is then formed by the combination of this precolumn with a main column. The feed air in this case is passed into the precolumn. From the bottom of the precolumn the purge fraction is taken off in the liquid state. At the top of the precolumn gas is taken off at least one theoretical or actual plate above the air feed and passed into the lower region of the main column. The oxygen-enriched fraction is then taken off from the bottom of the main column. In the case of rebuilding a conventional plant, the main column is part of the existing rectification system. Via the earlier feed air line, the overhead gas of the precolumn is passed into the main column and the oxygen-enriched fraction can be taken off via the earlier bottom liquid line which is already present. The retrofitting can therefore be effected by providing a precolumn for retaining low-volatility contaminants such as N2O. This method can also be expedient in the construction of a new air fractionation plant, for example- if a particularly low construction height is wanted.